Image sensors with diffractive lenses

ABSTRACT

An image sensor may include an array of imaging pixels. Each imaging pixel may have a photosensitive area that is covered by a respective diffractive lens to focus light onto the photosensitive area. The diffractive lenses may have a higher index of refraction than the surrounding materials. The diffractive lenses may be formed on an upper or lower surface of a planarization layer or may be embedded within the planarization layer. In some cases, multiple diffractive lenses may be formed over the imaging pixels. Some of the multiple diffractive lenses may have refractive indexes lower than the planarization layer such that the diffractive lenses defocus light. Focusing and defocusing diffractive lenses may be used to tune the response of the imaging pixels to incident light.

This application is a continuation of U.S. patent application Ser. No.15/719,174, filed Sep. 28, 2017, which is hereby incorporated byreference herein in its entirety. This application claims the benefit ofand claims priority to U.S. patent application Ser. No. 15/719,174,filed Sep. 28, 2017.

BACKGROUND

This relates generally to image sensors and, more particularly, to imagesensors having lenses to focus light.

Image sensors are commonly used in electronic devices such as cellulartelephones, cameras, and computers to capture images. In a typicalarrangement, an electronic device is provided with an array of imagepixels arranged in pixel rows and pixel columns. Each image pixel in thearray includes a photodiode that is coupled to a floating diffusionregion via a transfer gate. Each pixel receives incident photons (light)and converts the photons into electrical signals. Column circuitry iscoupled to each pixel column for reading out pixel signals from theimage pixels. Image sensors are sometimes designed to provide images toelectronic devices using a Joint Photographic Experts Group (JPEG)format.

Conventional image sensors sometimes include a color filter element anda microlens above each pixel. The microlenses of conventional imagesensors typically have curved surfaces and use refraction to focus lighton an underlying photodiode. However, these types of microlenses mayallow peripheral light to pass through the microlenses without beingfocused, leading to optical cross-talk.

It would therefore be desirable to provide improved lenses for imagesensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an illustrative electronic device thatmay include an image sensor in accordance with an embodiment.

FIG. 2 is a diagram of an illustrative pixel array and associatedreadout circuitry for reading out image signals from the pixel array inaccordance with an embodiment.

FIG. 3A is a cross-sectional side view of an illustrative focusingdiffractive lens with a greater index of refraction than the surroundingmedium in accordance with an embodiment.

FIG. 3B is a cross-sectional side view of an illustrative defocusingdiffractive lens with a lower index of refraction than the surroundingmedium in accordance with an embodiment.

FIGS. 4A and 4B are cross-sectional side views of illustrativediffractive lenses showing how the thickness of the diffractive lens maybe adjusted to change the response to incident light in accordance withan embodiment.

FIG. 5 is a cross-sectional side view of an illustrative image sensorwith diffractive lenses formed over the photosensitive area of eachpixel on a planarization layer in accordance with an embodiment.

FIG. 6 is a cross-sectional side view of an illustrative image sensorwith diffractive lenses formed over the photosensitive area of eachpixel within a planarization layer in accordance with an embodiment.

FIGS. 7A-7E are top views of illustrative diffractive lenses showingdifferent shapes for the diffractive lenses in accordance with anembodiment.

FIG. 8 is a cross-sectional side view of an illustrative image sensorwith more than one diffractive lens formed over the photosensitive areaof each pixel in accordance with an embodiment.

FIG. 9 is a cross-sectional side view of an illustrative image sensorwith diffractive lenses, a first layer having any refractive indexformed below the diffractive lenses, and a second layer having arefractive index that is lower than the refractive index of thediffractive lenses formed above and to the sides of the diffractivelenses in accordance with an embodiment.

FIG. 10 is a cross-sectional side view of an illustrative image sensorwith diffractive lenses, a first layer having any refractive indexformed above the diffractive lenses, and a second layer having arefractive index that is lower than the refractive index of thediffractive lenses formed below and to the sides of the diffractivelenses in accordance with an embodiment.

FIG. 11 is a cross-sectional side view of an illustrative image sensorwith diffractive lenses, a first layer having any refractive indexformed above the diffractive lenses, a second layer having a refractiveindex that is lower than the refractive index of the diffractive lensesformed to the sides of the diffractive lenses, and a third layer havingany refractive index formed below the diffractive lenses in accordancewith an embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention relate to image sensors with pixelsthat include diffractive lenses. An electronic device with a digitalcamera module is shown in FIG. 1. Electronic device 10 may be a digitalcamera, a computer, a cellular telephone, a medical device, or otherelectronic device. Camera module 12 (sometimes referred to as an imagingdevice) may include image sensor 16 and one or more lenses 29. Duringoperation, lenses 29 (sometimes referred to as optics 29) focus lightonto image sensor 16. Image sensor 16 includes photosensitive elements(e.g., pixels) that convert the light into digital data. Image sensorsmay have any number of pixels (e.g., hundreds, thousands, millions, ormore). A typical image sensor may, for example, have millions of pixels(e.g., megapixels). As examples, image sensor 16 may include biascircuitry (e.g., source follower load circuits), sample and holdcircuitry, correlated double sampling (CDS) circuitry, amplifiercircuitry, analog-to-digital (ADC) converter circuitry, data outputcircuitry, memory (e.g., buffer circuitry), address circuitry, etc.

Still and video image data from image sensor 16 may be provided to imageprocessing and data formatting circuitry 14 via path 27. Imageprocessing and data formatting circuitry 14 may be used to perform imageprocessing functions such as automatic focusing functions, depthsensing, data formatting, adjusting white balance and exposure,implementing video image stabilization, face detection, etc. Forexample, during automatic focusing operations, image processing and dataformatting circuitry 14 may process data gathered by phase detectionpixels in image sensor 16 to determine the magnitude and direction oflens movement (e.g., movement of lens 29) needed to bring an object ofinterest into focus.

Image processing and data formatting circuitry 14 may also be used tocompress raw camera image files if desired (e.g., to Joint PhotographicExperts Group or JPEG format). In a typical arrangement, which issometimes referred to as a system on chip (SOC) arrangement, camerasensor 16 and image processing and data formatting circuitry 14 areimplemented on a common integrated circuit. The use of a singleintegrated circuit to implement camera sensor 16 and image processingand data formatting circuitry 14 can help to reduce costs. This is,however, merely illustrative. If desired, camera sensor 14 and imageprocessing and data formatting circuitry 14 may be implemented usingseparate integrated circuits. If desired, camera sensor 16 and imageprocessing circuitry 14 may be formed on separate semiconductorsubstrates. For example, camera sensor 16 and image processing circuitry14 may be formed on separate substrates that have been stacked.

Camera module 12 may convey acquired image data to host subsystems 19over path 18 (e.g., image processing and data formatting circuitry 14may convey image data to subsystems 19). Electronic device 10 typicallyprovides a user with numerous high-level functions. In a computer oradvanced cellular telephone, for example, a user may be provided withthe ability to run user applications. To implement these functions, hostsubsystem 19 of electronic device 10 may include storage and processingcircuitry 17 and input-output devices 21 such as keypads, input-outputports, joysticks, and displays. Storage and processing circuitry 17 mayinclude volatile and nonvolatile memory (e.g., random-access memory,flash memory, hard drives, solid state drives, etc.). Storage andprocessing circuitry 17 may also include microprocessors,microcontrollers, digital signal processors, application specificintegrated circuits, or other processing circuits.

As shown in FIG. 2, image sensor 16 may include pixel array 20containing image sensor pixels 22 arranged in rows and columns(sometimes referred to herein as image pixels or pixels) and control andprocessing circuitry 24 (which may include, for example, image signalprocessing circuitry). Array 20 may contain, for example, hundreds orthousands of rows and columns of image sensor pixels 22. Controlcircuitry 24 may be coupled to row control circuitry 26 and imagereadout circuitry 28 (sometimes referred to as column control circuitry,readout circuitry, processing circuitry, or column decoder circuitry).Pixel array 20, control and processing circuitry 24, row controlcircuitry 26, and image readout circuitry 28 may be formed on asubstrate 23. If desired, some or all of the components of image sensor16 may instead be formed on substrates other than substrate 23, whichmay be connected to substrate 23, for instance, through wire bonding orflip-chip bonding.

Row control circuitry 26 may receive row addresses from controlcircuitry 24 and supply corresponding row control signals such as reset,row-select, charge transfer, dual conversion gain, and readout controlsignals to pixels 22 over row control paths 30. One or more conductivelines such as column lines 32 may be coupled to each column of pixels 22in array 20. Column lines 32 may be used for reading out image signalsfrom pixels 22 and for supplying bias signals (e.g., bias currents orbias voltages) to pixels 22. If desired, during pixel readoutoperations, a pixel row in array 20 may be selected using row controlcircuitry 26 and image signals generated by image pixels 22 in thatpixel row can be read out along column lines 32.

Image readout circuitry 28 may receive image signals (e.g., analog pixelvalues generated by pixels 22) over column lines 32. Image readoutcircuitry 28 may include sample-and-hold circuitry for sampling andtemporarily storing image signals read out from array 20, amplifiercircuitry, analog-to-digital conversion (ADC) circuitry, bias circuitry,column memory, latch circuitry for selectively enabling or disabling thecolumn circuitry, or other circuitry that is coupled to one or morecolumns of pixels in array 20 for operating pixels 22 and for readingout image signals from pixels 22. ADC circuitry in readout circuitry 28may convert analog pixel values received from array 20 intocorresponding digital pixel values (sometimes referred to as digitalimage data or digital pixel data). Image readout circuitry 28 may supplydigital pixel data to control and processing circuitry 24 over path 25for pixels in one or more pixel columns.

FIGS. 3A and 3B are cross-sectional side views of illustrativediffractive lenses that may be used in image sensors. As shown in FIG.3A, a diffractive lens 42 may be formed in a surrounding medium 44. Thesurrounding material 44 may be formed from a first material that has afirst index of refraction (n1). Diffractive lens 42 may be formed from asecond material that has a second index of refraction (n2). In theexample of FIG. 3A, the index of refraction of the lens may be greaterthan the index of refraction of the surrounding material (i.e., n2>n1).This results in incident light being focused towards a focal point. Inthis arrangement, diffractive lens 42 acts as a convex lens.

Lens 42 may be transparent to incident light. Therefore, some light maypass through the lens without being focused. For example, incident light46-1 may pass through the center of diffractive lens 42. Thecorresponding light 46-2 on the other side of the diffractive lens maytravel in the same direction as incident light 46-1. In contrast,incident light at the edge of diffractive lens 42 may be redirected dueto diffraction. For example, incident light 46-3 may pass by the edge ofdiffractive lens 42. The light may be redirected such that the outputlight 46-4 travels at an angle 48 relative to the incident light 46-3.In other words, the diffractive lens redirects the light at the edge ofthe lens using diffraction.

Diffraction occurs when a wave (such as light) encounters an obstacle.When light passes around the edge of an object, it will be bent orredirected such that the direction of the original incident lightchanges. The amount and direction of bending depends on numerousfactors. In an imaging sensor, diffraction of light can be used (withdiffractive lenses) to redirect incident light in desired ways (i.e.,focusing incident light on photodiodes to mitigate optical cross-talk).

In the example of FIG. 3A, diffractive lens 42 has an index ofrefraction greater than the index of refraction of the surroundingmedium 44. This causes incident light to be focused towards a focalpoint. However, this example is merely illustrative and otherembodiments may be used.

As shown in FIG. 3B, a diffractive lens 50 may be formed in asurrounding medium 52. The surrounding material 52 may be formed from afirst material that has a first index of refraction (n1). Diffractivelens 50 may be formed from a third material that has a third index ofrefraction (n3). In the example of FIG. 3B, the index of refraction ofthe lens may be less than the index of refraction of the surroundingmaterial (i.e., n1>n3). This results in incident light 46 beingdefocused. In this arrangement, diffractive lens 50 acts as a concavelens.

Lens 50 may be transparent to incident light. Therefore, some light maypass through the lens without being focused. For example, incident light46-1 may pass through the center of diffractive lens 50. Thecorresponding light 46-2 on the other side of the diffractive lens maytravel in the same direction as incident light 46-1. In contrast,incident light at the edge of diffractive lens 50 may be redirected dueto diffraction. For example, incident light 46-3 may pass by the edge ofdiffractive lens 50. The light may be redirected such that the outputlight 46-4 travels at an angle 54 relative to the incident light 46-3.In other words, the diffractive lens redirects the light at the edge ofthe lens using diffraction.

In addition to the refractive indexes of the diffractive lens and thesurrounding material, the thickness of the diffractive lens may alsoaffect the response of incident light to the diffractive lens. FIGS. 4Aand 4B show illustrative diffractive lenses used to focus incident light(as in FIG. 3A, for example). As shown in FIG. 4A, a diffractive lens 42may be formed in a surrounding medium 44. The surrounding material 44may be formed from a first material that has a first index of refraction(n1). Diffractive lens 42 may be formed from a second material that hasa second index of refraction (n2). In the example of FIG. 4A, the indexof refraction of the lens may be greater than the index of refraction ofthe surrounding material (i.e., n2>n1). This results in the light beingfocused to a focal point.

In particular, incident light 46-3 may pass by the edge of diffractivelens 42. The light may be redirected such that the output light 46-4travels at an angle 48-1 relative to the incident light 46-3. This anglemay be dependent upon the thickness 56 of diffractive lens 42. In theexample of FIG. 4A, thickness 56 is associated with an angle ofdiffraction of 48-1. Diffractive lens 42 in FIG. 4A may have arelatively large thickness and, accordingly, a relatively large angle ofdiffraction 48-1.

In contrast, diffractive lens 42 in FIG. 4B may have a relatively smallthickness and a relatively small angle of diffraction 48-2. As shown inFIG. 4B, a diffractive lens 42 may be formed in a surrounding medium 44.The surrounding material 44 may be formed from a first material that hasa first index of refraction (n1). Diffractive lens 42 may be formed froma second material that has a second index of refraction (n2). In theexample of FIG. 4B, the index of refraction of the lens may be greaterthan the index of refraction of the surrounding material (i.e., n2>n1).This results in the light being focused to a focal point. In particular,the light at the edge of the diffractive lens may be redirected suchthat the output light 46-4 travels at an angle 48-2 relative to theincident light 46-3. This angle may be dependent upon the thickness 58of diffractive lens 42. Because thickness 58 in FIG. 4B is less thanthickness 56 in FIG. 4A, angle 48-2 in FIG. 4B is less than angle 48-1in FIG. 4A.

Diffractive lenses 42 in FIGS. 4A and 4B have the same length and width.However, the length and width of diffractive lenses may also be adjustedto alter the response of incident light 46.

This shows how diffractive lenses may be used to redirect incident lightin desired ways. The refractive indexes of the lens and surroundingmaterial may be altered to customize the response of incident light.Additionally, the thickness, length, and width, of the diffractive lensmay be altered to customize the response of incident light.

FIG. 5 is a cross-sectional side view of an illustrative image sensorwith diffractive lenses. Image sensor 16 may include first and secondpixels such as Pixel 1 and Pixel 2. Pixel 1 and Pixel 2 may includephotosensitive regions 62 formed in a substrate such as siliconsubstrate 60. For example, Pixel 1 may include an associatedphotosensitive region such as photodiode PD1, and Pixel 2 may include anassociated photosensitive region such as photodiode PD2. Diffractivelenses 64 may be formed over photodiodes PD1 and PD2 and may be used todirect incident light towards photodiodes PD1 and PD2. An additionalanti-reflective coating 108 (sometimes referred to as a diffractive lensanti-reflective coating) may be formed on one or more surfaces ofdiffractive lenses 64. The additional anti-reflective coating 108 mayoptionally be applied to any of the diffractive lenses of FIGS. 3-11.

Color filters such as color filter elements 66 may be interposed betweendiffractive lenses 64 and substrate 60. Color filter elements 66 mayfilter incident light by only allowing predetermined wavelengths to passthrough color filter elements 66 (e.g., color filter 66 may only betransparent to the certain ranges of wavelengths). Color filter elements66 may be part of a color filter array formed on the back surface ofsubstrate 60. A respective diffractive lens 64 may cover each colorfilter element 66 in the color filter array. Light can enter from theback side of the image pixels through diffractive lenses 64. While inFIG. 5 image sensor 16 is a back-side illuminated image sensor, imagesensor 16 may instead be a front-side illuminated image sensor ifdesired. Photodiodes PD1 and PD2 may serve to absorb incident lightfocused by diffractive lenses 64 and produce pixel signals thatcorrespond to the amount of incident light absorbed.

Color filters 66 may include green filters, red filters, blue filters,yellow filters, cyan filters, magenta filters, clear filters, infraredfilters, or other types of filters. As an example, a green filter passesgreen light (e.g., light with wavelengths from 495 nm to 570 nm) andreflects and/or absorbs light out of that range (e.g., the green filterreflects red light and blue light). An example of a color filter arraypattern that may be used is the GRBG (green-red-blue-green) Bayerpattern. In this type of configuration, the color filter array isarranged into groups of four color filters. In each group, two of thefour color filters are green filters, one of the four color filters is ared filter, and the remaining color filter is a blue filter. If desired,other color filter array patterns may be used.

A layer (sometimes referred to as a planarization layer, passivationlayer, dielectric layer, film, planar film, or planarization film) maybe interposed between color filter elements 66 and diffractive lenses64. Planarization layer 68 may be formed across the entire array ofimaging pixels in image sensor 16. Cladding 70 may cover diffractivelenses 64 on the other side of planarization layer 68. In other words,diffractive lenses 64 may have first and second opposing sides withplanarization layer 68 formed on the first side and cladding 70 formedon the second side.

Diffractive lenses 64 may be formed from any desired material. It may bedesirable for diffractive lenses 64 to be transparent and formed from amaterial with a higher refractive index than the surrounding materials.Diffractive lenses 64 may sometimes be formed from silicon nitride (witha refractive index of approximately 1.9). In general, diffractive lenses64 may have any desired index of refraction (e.g., between 1.8 and 2.0,between 1.6 and 2.2, between 1.5 and 2.5, between 1.5 and 2.0, more than1.3, more than 1.6, more than 1.8, more than 2.0, less than 2.0, lessthan 1.8, etc.). Planarization layer 68 may also be transparent andformed from a material with any desired refractive index (e.g., a lowerrefractive index than diffractive lenses 64). Planar layer 68 may beformed from a material with a refractive index between 1.3 and 1.5,between 1.2 and 1.8, greater than 1.3, or any other desired refractiveindex. Cladding 70 may be formed from any desired material (i.e., air,the same material as planar film 68, etc.). Cladding 70 may also have adifferent (e.g., lower) refractive index than diffractive lenses 64.

Diffractive lenses 64 may have a higher index of refraction than thesurrounding materials (cladding 70 and planar film 68). Accordingly,light passing by the edge of diffractive lenses may be focused towardsthe photodiodes of the pixels. FIG. 5 shows incident light 46 beingfocused towards photosensitive areas PD1 and PD2 by diffractive lenses64. Focusing incident light in this way may reduce optical cross-talkbetween pixels.

As previously discussed, the refractive indexes of the diffractivelenses and surrounding materials, as well as the dimensions of thediffractive lenses, may be altered to customize the response to incidentlight. Additionally, the distance 72 between each diffractive lens maybe altered to change the response of incident light.

In some embodiments, the diffractive lens over each pixel in the pixelarray may be the same. However, in other embodiments different pixelsmay have unique diffractive lenses to further customize the response toincident light.

In FIG. 5, diffractive lenses 64 are formed on planar film 68, with acladding 70 formed over the diffractive lenses. In this embodiment,diffractive lenses 64 are mounted on the top surface of planarizationlayer 68. This example is merely illustrative, and other arrangementsmay be used if desired. For example, FIG. 6 shows an illustrative imagesensor 16 with diffractive lenses 64 embedded within layer 68. As shown,diffractive lenses 64 may be surrounded on all sides by planarizationlayer 68. Planarization layer 68 may still have a lower refractive indexthan diffractive lenses 64 to ensure incident light is focused onto thephotosensitive areas. Diffractive lenses 64 may be embedded within layer68 using deposition and etching or Lift-off steps. For example, a firstplanar portion of layer 68 may be deposited. Next, the material ofdiffractive lenses 64 may be deposited and etched (or removed usingLift-off steps) to form diffractive lenses of the desired shape.Finally, an additional portion of layer 68 may be deposited to fill thespace between diffractive lenses and cover the diffractive lenses suchthat layer 68 has a planar top surface.

As previously mentioned, each diffractive lens 64 may have any desiredshape. FIGS. 7A-7E are top views of illustrative diffractive lenses withdifferent shapes. As shown in FIG. 7A, diffractive lens 64 may have arectangular (or square) shape. As shown in FIG. 7B, diffractive lens 64may be formed from a shape with curved edges such as a circle or oval.In the embodiments of FIGS. 7A and 7B, diffractive lens 64 does not haveany openings. However, these examples are merely illustrative. As shownin FIG. 7C, diffractive lens 64 may have one or more openings such thatthe lens is ring-shaped. As shown in FIG. 7D, the diffractive lens doesnot have to be a regular shape. FIG. 7D shows an irregularly shapeddiffractive lens. The diffractive lens may include one or more planarsides (i.e., 64-1), one or more curved sides that curve outward (i.e.,64-2), and/or one or more curved sides that curve inward (i.e., 64-3).Finally, as shown in FIG. 7E, the diffractive lens may be split intomore than one section. The diffractive lens may have two or moreseparately formed vertical sections or two or more separately formedhorizontal sections.

In the embodiments of FIGS. 5 and 6, one diffractive lens is formed overeach pixel. These examples are merely illustrative. If desired, morethan one diffractive lens may be formed over each image pixel. FIG. 8shows an illustrative image sensor 16 with multiple diffractive lensesover each pixel. In some embodiments, each diffractive lens may have arefractive index greater than the refractive index of the surroundinglayer 68 (i.e., each diffractive lens may be a focusing lens). In otherembodiments, each diffractive lens may have a refractive index lowerthan the refractive index of the surrounding layer 68 (i.e., eachdiffractive lens may be a defocusing lens). In yet other embodiments,one or more diffractive lenses may have a refractive index lower thanthe refractive index of the surrounding layer 68 whereas one or morediffractive lenses may have a refractive index greater than therefractive index of the surrounding layer (i.e., there may be one ormore defocusing lenses and one or more focusing lenses).

FIGS. 5 and 6 show illustrative arrangements for a diffractive lens. InFIG. 5, diffractive lenses 64 are mounted on the top surface ofplanarization layer 68 (and diffractive lenses 64 may have a higherindex of refraction than planarization layer 68) and in FIG. 6diffractive lenses 64 are embedded within planarization layer 68 (anddiffractive lenses 64 may have a higher index of refraction thanplanarization layer 68). However, other arrangements may be used fordiffractive lenses 64 and the surrounding layers if desired.

FIGS. 9-11 are cross-sectional side views showing possible arrangementsfor layers adjacent to diffractive lenses 64 in image sensor 16. Asshown in FIG. 9, a first layer 82 may be formed underneath (below)diffractive lenses 64. Layer 82 may have any desired refractive index(e.g., greater than, less than, or equal to the index of refraction ofdiffractive lenses 64). A second layer 84 may be formed above (over) andto the sides of diffractive lenses 84 (e.g., a first portion of layer 84is formed above the upper surfaces of diffractive lenses 64 and a secondportion of layer 84 is interposed between the side surfaces of adjacentdiffractive lenses 64). Layer 84 may have a refractive index that isless than the refractive index of diffractive lenses 64. Layers 82 and84 may be transparent and may be formed from any desired materials.Layers 82 and 84 may be formed from the same materials or differentmaterials. Layers 82 and 84 may sometimes be referred to asplanarization layers, dielectric layers, or cladding layers.

As shown in FIG. 10, a first layer 86 may be formed over diffractivelenses 64. Layer 86 may have any desired index of refraction (e.g.,greater than, less than, or equal to the index of refraction ofdiffractive lenses 64). A second layer 88 may be formed under and to thesides of diffractive lenses 64 (e.g., a first portion of layer 88 isformed below the lower surfaces of diffractive lenses 64 and a secondportion of layer 84 is interposed between the side surfaces of adjacentdiffractive lenses 64). Layer 88 may have a refractive index that isless than the refractive index of diffractive lenses 64. Layers 86 and88 may be transparent and may be formed from any desired materials.Layers 86 and 88 may be formed from the same materials or differentmaterials. Layers 86 and 88 may sometimes be referred to asplanarization layers, dielectric layers, or cladding layers.

In yet another arrangement shown in FIG. 11, a first layer 90 may beformed over diffractive lenses 64. Layer 90 may have any desired indexof refraction (e.g., greater than, less than, or equal to the index ofrefraction of diffractive lenses 64). A second layer 92 may be formedbetween diffractive lenses 64 (e.g., layer 92 may be interposed betweenthe side surfaces of adjacent diffractive lenses 64). Layer 92 may havean index of refraction that is less than the index of refraction ofdiffractive lenses 64. A third layer 94 may be formed under diffractivelenses 64. Layer 94 may have any desired index of refraction (e.g.,greater than, less than, or equal to the index of refraction ofdiffractive lenses 64). Layers 90, 92, and 94 may be transparent and maybe formed from any desired materials. Layers 90, 92, and 94 may beformed from the same materials or different materials. Layers 90, 92,and 94 may sometimes be referred to as planarization layers, dielectriclayers, or cladding layers.

In the embodiments of FIGS. 5, 6, 7A-7E, and 8-11, at least onediffractive lens is formed over each pixel. These diffractive lenses maytake the place of any other per-pixel lenses. For example, nomicrolenses may be present over each pixel that have curved uppersurfaces. No microlenses may be present over each pixel that userefraction to focus light.

In various embodiments, an image sensor may have a plurality of imagingpixels that each includes a photodiode, a color filter element formedover the photodiode, and a diffractive lens formed over the color filterelement. The diffractive lens of each imaging pixel may have a planarupper surface and a planar lower surface. No microlens with a curvedsurface may be formed over the diffractive lens of each pixel. Thediffractive lens may include silicon nitride. The image sensor may alsoinclude a planarization layer that is formed over the plurality ofimaging pixels. Each diffractive lens may be formed on an upper surfaceof the planarization layer. Each diffractive lens may be embedded withinthe planarization layer. The planarization layer may have a first indexof refraction, the diffractive lens for each imaging pixel may have asecond index of refraction, and the second index of refraction may begreater than the first index of refraction. There may be a gap betweeneach diffractive lens and respective adjacent diffractive lenses.

In various embodiments, an imaging pixel may include a photosensitivearea, a color filter element formed over the photosensitive area, aplanarization layer formed over the color filter element, and adiffractive lens formed over the color filter element. The diffractivelens may be transparent, the diffractive lens may have first and secondopposing surfaces, the first and second surfaces of the diffractive lensmay be planar, the diffractive lens may have a first index ofrefraction, and the planarization layer may have a second index ofrefraction that is lower than the first index of refraction.

The planarization layer may have first and second opposing surfaces andthe first and second surfaces of the planarization layer may be parallelto the first and second surfaces of the diffractive lens. Thediffractive lens may be formed on an upper (or lower) surface of theplanarization layer. The diffractive lens may be embedded within theplanarization layer. Light incident on a central portion of the imagingpixel may pass through the diffractive lens without being redirected andlight incident on an edge portion of the imaging pixel may be redirectedby the diffractive lens towards the photosensitive area. The imagingpixel may also include an additional diffractive lens formed over (orunder) the diffractive lens. The additional diffractive lens may have athird index of refraction. The second index of refraction may be greaterthan the third index of refraction. The second index of refraction maybe less than the third index of refraction.

In various embodiments, an image sensor may include a plurality ofimaging pixels that each includes a photosensitive area and a planardiffractive lens formed over the photosensitive area that focusesincident light onto the photosensitive area. The planar diffractive lensof each imaging pixel may be surrounded by at least one layer and theplanar diffractive lens of each imaging pixel may have a greater indexof refraction than the at least one layer. The planar diffractive lensmay include silicon nitride.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention.

What is claimed is:
 1. An image sensor comprising a plurality of imagingpixels, wherein each imaging pixel of the plurality of imaging pixelscomprises: a photodiode; and a diffractive lens formed over thephotodiode, wherein the diffractive lens of each imaging pixel is formedfrom a first material that has a first index of refraction, whereinthere is a gap between each diffractive lens and respective adjacentdiffractive lenses, and wherein a layer that is formed from a secondmaterial having a second index of refraction that is different than thefirst index of refraction fills each gap and extends continuouslybetween each diffractive lens and respective adjacent diffractivelenses.
 2. The image sensor defined in claim 1, wherein the diffractivelens of each imaging pixel has a planar upper surface and a planar lowersurface.
 3. The image sensor defined in claim 2, wherein no microlenswith a curved surface is formed over the diffractive lens of each pixel.4. The image sensor defined in claim 1, wherein the diffractive lens ofeach imaging pixel comprises silicon nitride.
 5. The image sensordefined in claim 1, further comprising: a planarization layer that isformed over the plurality of imaging pixels.
 6. The image sensor definedin claim 5, wherein each diffractive lens is formed on a surface of theplanarization layer.
 7. The image sensor defined in claim 6, wherein thelayer that is formed from the second material is formed on the surfaceof the planarization layer and wherein only the layer that is formedfrom the second material and the diffractive lenses are formed in directcontact with the surface of the planarization layer.
 8. The image sensordefined in claim 1, wherein the layer that is formed from the secondmaterial comprises a planarization layer and wherein each diffractivelens is embedded within the planarization layer.
 9. An imaging pixelcomprising: a photosensitive area; a first diffractive lens formed overthe photosensitive area, wherein the first diffractive lens istransparent, wherein the first diffractive lens has first and secondopposing surfaces, wherein the first and second surfaces of the firstdiffractive lens are planar, and wherein the diffractive lens has afirst index of refraction; a layer of material formed over the firstdiffractive lens that has a second index of refraction that is differentthan the first index of refraction; and a second diffractive lens formedover the first diffractive lens, wherein a portion of the layer ofmaterial is interposed between the first diffractive lens and the seconddiffractive lens.
 10. The imaging pixel defined in claim 9, wherein thefirst diffractive lens is embedded within the layer of material.
 11. Theimaging pixel defined in claim 10, wherein the second diffractive lensis embedded within the layer of material.
 12. The imaging pixel definedin claim 9, wherein the second diffractive lens has a third index ofrefraction that is different than the first and second indices ofrefraction.
 13. The imaging pixel defined in claim 12, wherein the thirdindex of refraction is greater than the first index of refraction. 14.The imaging pixel defined in claim 12, wherein the third index ofrefraction is less than the first index of refraction.
 15. The imagingpixel defined in claim 9, further comprising: a color filter elementformed over the photosensitive area.
 16. An image sensor comprising aplurality of imaging pixels, wherein each imaging pixel of the pluralityof imaging pixels comprises: a photosensitive area; a diffractive lensformed over the photosensitive area, wherein the diffractive lens has anupper surface, a lower surface, and edge surfaces and wherein thediffractive lens has a first index of refraction; and a layer ofmaterial having a second index of refraction that is different than thefirst index of refraction, wherein the edge surfaces and at least one ofthe upper surface and the lower surface of the diffractive lens are indirect contact with the layer of material.
 17. The image sensor definedin claim 16, wherein the lower surface of the diffractive lens is indirect contact with the layer of material.
 18. The image sensor definedin claim 16, wherein the upper surface of the diffractive lens is indirect contact with the layer of material.
 19. The image sensor definedin claim 16, wherein the lower surface and the upper surface of thediffractive lens are in direct contact with the layer of material. 20.The image sensor defined in claim 16, wherein the diffractive lenscomprises silicon nitride.